1. Field of the Invention
The invention relates to a method for selectively growing semiconductor crystals, and more particularly to a method for selectively growing semiconductor crystals by which atomic ordering of a semiconductor layer or formation of natural super lattice (NSL) is controlled to thereby greatly change the material properties of a semiconductor layer such as energy gap, optical anisotropy and electrical conductivity, relative to prior methods.
2. Description of the Related Art
Many attempts have been made to form a striped pattern with SiO.sub.2 or SiNx on a semiconductor substrate in the field of a chemical compound semiconductor including group III-V species, to thereby selectively grow a semiconductor layer on non-patterned regions by various methods such as MOVPE, MBE or VPE. For instance, there is reported one such attempt in the 5th International Conference on Indium Phosphide and related Materials, 1993, pages 44-47 and 52-55. In such a selective growth, it is possible to vary the composition of a semiconductor layer at selective growth regions in accordance with a difference of a degree of removal from and adsorption into a surface of atoms and molecules flying thereto, or a difference of a migration or a diffusion length of atoms and molecules flying thereto, which atoms and molecules contribute to the growth of a semiconductor layer. FIGS. 1 and 1A show the dependency on the width of a mask of the energy gap of an InGaAs semiconductor layer in the selective MOVPE growth. Indium (In) atoms have a longer diffusion length than gallium atoms, and hence the indium atoms have a higher arrival probability than the gallium atoms. In other words, the In atoms can arrive at a selective growth layer earlier than the Ga atoms. Accordingly, the narrower a mask width is, the higher dependency probability with which a selective growth region is dependent on In atoms the selective growth region has, and thereby the energy gap is decreased, namely the content of In is increased.
The selective growth process as mentioned above makes it possible to form regions in a substrate, which regions have different two-dimensional energy gaps. Hence, the selective growth technique is applied to various optical or electronic devices.
The energy gap change of the selective growth layer is due to an increase of the probability with which a particular atom contributes to growth of a selective growth region. For widening the applications of the selective growth process, it is required to two-dimensionally induce a larger composition change or a larger energy gap change. However, an increase in the number of particular atoms causes a large lattice mismatching to the substrate, and hence it is impossible to form a monocrystal epitaxial layer having good quality. Thus, the prior technique cannot practically provide a large energy gap change.
In addition, the prior technique deals only with energy gap change. However, if it would be possible to two-dimensionally control material properties such as optical anisotropy and electrical conductivity in a substrate, the selective growth technique could be utilized in wider field such as an optical integrated circuit.